1. Field of the Invention
The invention relates to switching of optical signals and more particularly to an acousto-optic matrix switch for coupling between a plurality of fiber optic cables.
2. Description of the Prior Art
Optical fibers are widely used for communications because of their extremely wide information bandwidth and immunity to ground loops and electromagnetic interference. Many commercial and military applications utilize optical fibers for interconnections between high speed digital devices or to connect between sensors and signal processing systems. Many such fiber optic data communication systems require a dynamically reconfigurable data network. Optical switches are essential in order to construct such data networks with wide bandwidth and high reliability. The conventional structure for accomplishing this task involves mechanical switches or non-mechanical switches using optical waveguides wherein the optical signals are routed through a crystal and switched using an electro-optic effect. Mechanical switches are limited in speed of operation and bandwidth. The optical waveguide switches suffer from material instabilities and temperature sensitivity. Further, the structures heretofore employed have required a plurality of switching elements, typically n.sup.2 elements for an n.times.n matrix, operated in a blocking mode, thereby resulting in excessive insertion loss and cross-talk.
Multi-position signal switches utilizing acousto-optic elements are known in the prior art. For example, U.S. Pat. No. 4,530,573 describes a system in which a Bragg cell is acoustically modulated in accordance with one or more preselected control frequencies. A light beam is deflected by an amount dependent on the acoustically modulated frequency. A plurality of photodetectors are positioned to intercept the deflected beam at its respective possible deflection angles and to convert the optical signals so received into electrical output signals. Thus, by the choice of control frequency, a signal corresponding to the input signal can be caused to appear at the output of one or more photodiodes.
The aforesaid invention discloses a Bragg cell comprised of lithium niobate. A relatively low figure of merit of this material requires that active detector elements be utilized in order to distinguish sufficiently from the noise background. Further, an array of such detector elements is subject to cross-coupling and resultant cross-talk in the switch outputs.
Another type of switch is described in the article "Path-Independent Insertion Loss Optical Space Switch", T. Shimoe, et al, Technical Digest, Sixth International Conference on Integrated Optics and Optical Fiber Communication, 19-22, Jan. 1987. Shimoe describes an n.times.n matrix switch comprised of n.sup.2 switch elements. In the prior art described therein, since the number of switch elements through which optical signals pass are path-dependent, both insertion loss and signal-to-cross-talk ratio are degraded. An improved switch described in the article provides a constant number of switch elements for any selected switch path, but requires a plurality of such elements resulting in excessive insertion losses. Further, this article does not disclose the use of acousto-optic switch elements.
The present invention provides an acousto-optic switch which requires no active optical waveguide devices to detect, switch, or regenerate an optical signal. It provides low fiber-to-fiber insertion loss, low cross-talk, high efficiency, and wide bandwidth. The components have a mature technology base and are commercially available.